[docs] [issue9101] reference json format in file formats chapter

[anatoly techtonik]

anatoly techtonik <techtonik at gmail.com> added the comment:

Awesome. Especially links to actual changesets. BTW, there are only backport commits - where is the notification about the main one?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue9101>
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